The leading cause of electrical injury in the workplace is accidental contact between crane booms and overhead power lines while a worker is touching the crane load. In response to this hazard, links have been developed which electrically isolate the load from the crane cable. These insulated links prevent electrical current flow from the boom through a worker guiding the crane load on the ground. Normally, such insulated links include a dielectric material that can support a heavy load and is also resistant to breakdown even when a large electrical potential is applied across it.
One of the problems associated with the present manufacture of insulated link devices is the unreliable vulcanizing process used in manufacturing the links. This process results in low manufacturing yields due to frequent cracking of the dielectric during the curing phase. This cracking is often caused by the high thermal mass of the metal parts of the link that are embedded in the dielectric. The process is also expensive because when the defective dielectric is revealed during electrical testing, the entire link unit must be scrapped.
For example, U.S. Pat. No. 2,897,257 to Ingram et al. discloses a conventionally constructed insulated link. The link includes two pairs of steel plates separated by a dielectric made of rubber or a rubber-like material. The dielectric is formed integrally of a single homogeneous mass of the insulating material cast or molded upon and around the assembled metal structure. A mold and fixture used in making the link are also disclosed, as is the process for constructing the link. The link has acceptable load-handling and dielectric properties, but the vulcanizing process used to make the link is time-consuming and labor intensive, and is therefore expensive and results in low yields.
A simpler, less expensive method of manufacturing such a link would be to separate the metal plates of the assembly from each other by a castable insulator made of a high strength, highly dielectric material and then molding a dielectric jacket around this core to form an outer casing. Those of ordinary skill in the art believe that such a construction could not avoid the formation of internal air pockets, resulting in the possibility of breakdown of the insulation in the presence of a high voltage across the link. A link employing such construction and avoiding the formation of such air pockets or shrinkage cracks would provide a strong, highly dielectric link that would be easier and less expensive to manufacture than conventional links.